In the industrial printing market, there exists a need for a low cost, high performance printing system that enables imaging of data onto a variety of porous and/or non-porous substrates. The technologies most often employed in the industrial printing market are continuous inkjet and drop-on-demand inkjet.
With regard to continuous inkjet systems, solvent based inks are typically the inks of choice to print on the various substrates required. The ink composition is typically based on solvents, e.g., methyl ethyl ketone (“MEK”) and/or alcohol, and one or more solvent-soluble colorants. Inks for continuous inkjet systems are generally conductive since an electrostatic charging device is used to assist in directing the continuous stream of ink droplets. In such continuous inkjet systems, the ink droplets to be imaged are jetted from a nozzle as a continuous stream and directed to a substrate. The un-imaged or unused ink droplets are recycled back into the bulk ink feed system. Due to the high volatility nature of solvent-based inks and the recycling of the un-imaged ink droplets, a make-up solution comprising an effective concentration of the solvent(s) is typically used to compensate for the loss in solvent(s) during printing.
Due to the high volatility of continuous inkjet inks, a plurality of problems is often encountered with continuous inkjet technology. Volatile inks tend to generate volatile organic compounds (“VOCs”) that are not environmentally friendly upon the loss of the solvent(s). Loss of the solvent(s) from such volatile inks also tends to cause the ink to increase in viscosity. To compensate for the loss of solvents(s) and the resulting increase in viscosity, again, make-up solution is added. The addition of make-up solution increases the cost of operation. Furthermore, the variability in the physical properties of the ink due to the loss of solvent(s) and the compensation by the make-up solution creates variability in quality of the printed image. As a result of these factors, and due to the relative complexity of the technology more generally, continuous inkjet systems ultimately tend to be expensive to own and maintain.
To overcome the volatility problems related to solvent inks used in continuous inkjet system, and in the alternative, hot melt ink may be employed using a drop-on-demand printing system. In such hot melt ink printing system, the ink composition is typically based on low molecular weight waxes and oil soluble colorant(s). The ink is typically a solid at room temperature. During the imaging process, the ink is heated up above the melting point to an effective temperature where the ink is jettable. The molten ink is jetted from the nozzles of the print head and onto the desired substrate via a micro piezo-actuated device. The molten ink droplets freeze on top of the substrate to form the desired image. Since this is a solid ink system, there is no solvent being lost to evaporation. However, the printed images tend to suffer from a lack of image durability. The solid printed image sitting on top of the substrate tends to mar and scuff relatively easily when it comes into contact with various feeding rollers in the industrial printing environment, often rendering the image even illegible. Furthermore, the typical hot melt inkjet printing system does nothing to remedy the high system and maintenance cost problems that result primarily from the expensive low volume print head design and manufacturing and from the high power requirement to heat the system and print head(s).
Thus, although solid ink drop-on-demand technology resolves the volatility problem of continuous inkjet technology by not using solvents, the cost issues have not been resolved. In addition, hot melt ink technology tends to suffer from a durability problem of the printed image.
Recently, then, new drop-on-demand printing systems based on thermal inkjet technology available from Hewlett Packard® and others have been integrated for the industrial printing market. Such thermal inkjet technology is based on the disposable, inexpensive print head technology that has been commercially successful in the high volume office printing market for many years. Integrators of the thermal inkjet technology have been able to simultaneously address both issues of high system cost and maintenance cost.
In thermal ink jet technology, the ink composition is typically based on water and glycols. The colorants typically are water soluble dyes or water dispersible pigments. Because of the inherent water solubility of water-soluble dyes, the images printed from these inks suffer from a lack of waterfastness. Whereas, the use of water-based inks for thermal inkjet systems have found great commercial success in printing on porous or absorbent substrates. However, water based inks are notorious for requiring appropriate substrates to be selected for optimum print quality and when printed on non-porous substrates often suffer from low edge acuity, poor wetting, inadequate drying, coalescence in the halftone image, mottling, smudging, low optical density, poor adhesion to the substrate, lack of waterfastness, and other such problems. Furthermore, thermal inkjet technology is prone to long-term reliability issues such as nozzles clogging from dried ink at the orifices, kogation due to thermal degradation at the resistors, or corrosion due to oxidative problems.
In sum, there are several disadvantages to the aforementioned methods. Continuous inkjet systems may be expensive to own and maintain. Additionally, continuous inkjet inks might be highly volatile, might create print quality problems, might generate VOCs into the environment, and might add cost to the operation. Hot melt drop-on-demand inkjet systems may be able to resolve the issues related to volatility found in continuous inkjet systems. However, hot melt inkjet systems generally introduce printed images of low durability without resolving the cost issues. Thermal inkjet systems may be able to resolve the cost and ink volatility issues. However, thermal inkjet systems introduce problems with printing on non-porous substrates. Thus, the prior art described above teaches inks and ink systems that may be relatively inexpensive to operate, relatively non-volatile, or relatively successful in printing quality images on non-porous substrates, but does not teach inks and ink systems that are capable of achieving all three of these objectives. Aspects of the present invention fulfill these needs and provide further related advantages as described in the following disclosure.